Adaptive Radiation of the Hawaiian Silversword Alliance

Natural Hybridization

Last Modification: April 21, 2006

Biosystematic and cytogenetic analyses indicate
that hybrids among the group of Dubautia species with 13 pairs of chromosomes are
essentially fully fertile. Thus, where such species are sympatric, it is common to find
evidence of hybridization, sometimes resulting in spectacular swarms of recombinant types
representing every conceivable intermediate between the parental types. One superb example
of this type involves D. arborea and D.
ciliolata in a small area of sympatry in Waipahoehoe Gulch on Mauna Kea, Hawaii.
Interestingly, a form that is very similar to D. menziesii,
otherwise known only from Maui, has apparently become reproductively stabilized in the
lower part of this gulch and in at least one other area of sympatry on Mauna Kea. The
illustrations at the left show an array of shoots (above) and leaves (below) each taken
from from a different individual growing in the narrow zone of sympatry in this gulch.
(side by side comparison)

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Dubautia scabra is sympatric and hybridizes with several 13-paired species,
including D. ciliolata, D. linearis, D. menziesii(Fig. 1),D. platyphylla,
D. reticulata, and D.
waianapanapaensis. These hybrids can be readily recognized by their intermediate
nature, including pale lemon-yellow flowers and a chromosome number of 2n = 27.
The maximum meiotic chromosome pairing configuration is always the same, consisting of 12 pairs and 1 chair of three chromosomes. Pollen
stainability in these hybrids often ranges from 70-90%. Dubautia ciliolata and D.
scabra (Fig. 2) hybridize at countless sites of sympatry on the island of Hawaii. In
the saddle area between Mauna Kea and Mauna Loa is a particularly interesting site where D.
ciliolata is restricted to kipuka (islands) of a prehistoric, mostly a'a lava flow
and D. scabra is restricted to the surrounding 1935 flow of mostly pahoehoe lava.
Fig. 3 shows a large kipuka of older lava with shrubby vegetation and a tree surrounded by
lava of the more recent flow with sparse vegetation. A very small kipuka of the older
reddish-brown lava with a shrub of Dubautia ciliolata is visible in Fig. 4.
Hybrids occur primarily on the the perimeter of the recent lava flow, usually at
interfaces with the older substrate. A study of flavonoid compounds in hybrids suggests
that recombination beyond the F1 generation is occurring at this site.
(side by side comparison)

A striking example of a natural hybrid between congeneric species with very different
growth forms involves the mat-forming species, Dubautia scabra,
and the tree, D. reticulata. As different as the parents
are, the hybrids are comparatively fertile, being identical in chromosome configuration to
the D. scabra/D. ciliolata example discussed above. (side
by side comparison)

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The preceding examples involve hybrids with full or high fertility between species
that are relatively closely related. Among the more spectacular hybrids that occur
naturally is the one between Argyroxiphium sandwicense subsp. macrocephalum
(Fig. 1) with 14 pairs of chromosomes and Dubautia menziesii
(Fig. 2) with 13 pairs of chromosomes. In addition to the difference in chromosome number,
the genomes of these species are further differentiated by at least two reciprocal
chromosome translocations. This results in a common meiotic configuration of 9 pairs and 3 chains of three chromosomes, and a
concomitant reduction in fertility to approximately 9%, based on pollen stainability in F1
hybrids. Despite the tremendous morphological differences of the parents and the reduced
fertility of the F1 (Fig.3), backcross progeny are produced in nature. Some of
these have been inadvertently cultivated in Haleakala National park (Fig. 4) and several
others have been grown experimentally at the University of Hawaii-Manoa. One of the
experimentally grown plants that represented a backcross to Dubautia menziesii
flowered (Fig 5, note many heads were removed from this plant for chromosome analysis). It
had a simplified chromosome pairing configuration of 12 pairs and a chain of 3
chromosomes, was about 80% fertile, and was used to generate a vigorous second backcross
progeny of several individuals with Dubautia menziesii as the recurrent parent.
These individuals were remarkably uniform morphologically and one that flowered (Fig. 6)
exhibited 13 pairs of chromosomes and 99% pollen stainability (see summary in Fig. 7).
Some plants seen in the field closely approximate the cultivated backcross progeny of this
intergeneric hybrid combination and probably originated in the same manner. Specifically,
the type material of D. dolosa appears to represent such an unstabilized hybrid
product and is no longer equated with D. waianapanapaensis
which is geographically distinct and clearly a reproductively stabilized taxon. However,
the similarity of these plants suggests a possible hybrid origin for D.
waianapanapaensis. Indeed, the ease of recombination between such strikingly
differentiated plants as Argyroxiphium sandwicense subsp. macrocephalum
and Dubautia menziesii underscores the potential of hybridization in the
evolution of this and other plants, especially in Hawaii, where hybridization appears to
be a way of life. (side by side comparison)